EP1765752B1 - Method for separating phenol from streams containing phenol that have been obtained during the production of bisphenol a - Google Patents

Description

Bisphenols as condensation products of phenols and carbonyl compounds are starting materials or intermediates for the preparation of a variety of commercial products. Of particular industrial importance is the condensation product of the reaction between phenol and acetone, 2,2-bis (4-hydroxyphenyl) propane (bisphenol A, BPA). BPA serves as a starting material for the preparation of various polymeric materials such as polyarylates, polyetherimides, polysulfones and modified phenol-formaldehyde resins. Preferred fields of application are in the production of epoxy resins and polycarbonates.

The industrially relevant production methods for BPA are based on the acid-catalyzed reaction of phenol with acetone, wherein preferably a phenol-acetone ratio of> 5: 1 is set in the reaction. As acid catalysts, homogeneous as well as heterogeneous Bronsted or Lewis acids can be used, for example strong mineral acids such as hydrochloric or sulfuric acid. Gel-form or macroporous sulfonated crosslinked polystyrene resins (acidic ion exchangers) are preferably used. The following statements relate to a process using acidic ion exchangers as catalysts. These can be mono- or heterodisperse.

To achieve high selectivities, the reaction of phenol with acetone is carried out in the presence of suitable mercapto compounds as cocatalysts. These can either be dissolved homogeneously in the reaction solution or fixed via ionic or covalent bonds to the sulfonated polystyrene matrix. The reaction unit is a bed of layer or fluidized bed, which is flowed through or down, or a column in the manner of a reactive distillation column.

The selectivity of the reaction as well as the long-term stability of the catalyst is significantly influenced by the quality of the raw materials phenol and acetone used. In particular, for the production of BPA as a raw material for high-quality plastics such as polycarbonate therefore very high purity requirements are placed on the raw materials used phenol and acetone. In this case, purities of> 99.95% by weight for phenol and> 99.90% by weight for acetone with simultaneously low impurity levels (S <0.5 ppm, Fe <1 ppm) are typically positive for achieving high product purities and to minimize catalyst deactivation. So will in EP-A-876,319 described that commercial phenol is freed of interfering impurities by treatment with molecular sieve and thereby ensures better usability in a process for BPA production

Adapted description

becomes. EP-A-680 913 describes the use of modified acidic ion exchangers to remove hydroxyacetone from phenol for BPA synthesis.

EP 812 815 A2 describes an integrated process for the preparation of bisphenol wherein phenol and ketone react in the presence of an ion exchanger as a catalyst to a mixture containing a bisphenol. EP 812 815 A2 However, does not describe working up of residual resin in which even larger amounts of phenol are included, which leads to a loss of resources by disposal.

In the reaction of phenol with acetone in the presence of acidic catalysts and mercapto compounds as cocatalysts, a product mixture is formed which, in addition to unreacted phenol and optionally acetone, primarily contains BPA and water. In addition, small amounts of typical by-products of the condensation reaction occur, such as 2- (4-hydroxyphenyl) -2- (2-hydroxyphenyl) propane (o, p-BPA), substituted indanes, hydroxyphenylindanols, hydroxyphenyl chromans, substituted xanthenes, and higher condensed compounds with three or more phenyl rings in the molecular skeleton.

The said by-products as well as water, phenol and acetone impair the suitability of BPA for the production of polymers and must be separated by suitable methods. In particular, for the production of polycarbonate high purity requirements are placed on the raw material BPA.

The work-up and purification of BPA is carried out by a multistage cascade of suitable purification processes such as, for example, suspension crystallization, melt crystallization, distillation and / or desorption. In a technically preferred embodiment, the separation of BPA from the reaction mixture takes place in the form of an approximately equimolar crystalline adduct with phenol by cooling the reaction mixture with crystallization of the BPA / phenol adduct. The crystallization is preferably carried out as suspension crystallization. The BPA / phenol adduct crystals are then separated from the liquid phase by a suitable apparatus for solid-liquid separation such as rotary filters or centrifuges and fed for further purification. The adduct crystals obtained in this way typically have a purity of> 99% by weight BPA based on BPA and the secondary components with a phenol content of about 40% by weight, based on the total adduct crystal amount. By washing with suitable solutions, which typically contain one or more components from the group consisting of acetone, water, phenol, BPA and secondary components, the adduct crystals can be freed of superficially adhering impurities. The BPA-phenol adduct crystals obtained following the above-described suspension crystallization of the reaction solution and solid-liquid separation are fed to further purification steps, wherein the separation of phenol is carried out and optionally by the use of suitable purification operations (suspension crystallization, layer crystallization, extraction, distillation). a reduction in the concentration of minor components is achieved.

The liquid stream (mother liquor) obtained in the solid-liquid separation contains phenol, BPA, water formed in the reaction, unreacted acetone and is enriched in the secondary components typically produced during BPA production. This mother liquor stream is recycled in a preferred embodiment in the reaction unit. In order to maintain the catalytic activity of the acidic ion exchangers, water formed beforehand is removed completely or partially by distillation, any acetone still present being wholly or partly removed from the mother liquor. The dehydrated reaction stream thus obtained is supplemented with phenol and acetone and returned to the reaction unit. Alternatively, water and acetone can be completely or partially removed by distillation before carrying out the suspension crystallization of the BPA-phenol adduct. In the said distillation steps, a portion of the phenol present in the reaction solution can be separated by distillation.

In such a circulation procedure occurs as a problem that by-products of BPA production are enriched in the circulation stream, which can adversely affect the purity of BPA in the suspension crystallization and can lead to deactivation of the catalyst system. In order to avoid an excessive accumulation of secondary components in the circulation stream, a subset of the mother liquor mixture must be fed out of the system. The amount of secondary components removed in this way from the process must correspond in the equilibrium state to the amount of secondary components formed in the reaction. This outfeed is typically carried out in such a way that a partial amount of the mother liquor is taken from the circulation stream, wherein previously optionally formed reaction water, unreacted acetone and aliquots of phenol can be removed by distillation. The composition of the mother liquor at this point and thus also the composition of the extracted partial amount typically consists of 70 to 90% by weight of phenol, 3 to 15% by weight of BPA and 3 to 15% by weight of secondary components and isomers which are present in the Reaction were formed. Since ultimately only the last-mentioned proportion of secondary components has to be removed from the process, the discharged quantity is subjected to further work-up steps in order to minimize material losses.

In a simple embodiment, this phenol is distilled off to a residual content of <10 wt .-%, so that a residual resin with a content of <10 wt .-% phenol, 15 to 85 wt .-% BPA and 15 to 85 wt. -% secondary components is obtained, which is removed from the process and disposed of, for example, by incineration or landfill.

In another embodiment, a portion of the BPA contained in the recirculated amount is recovered by distilling off part of the phenol from the withdrawn partial stream and supplying the resulting enriched solution to suspension crystallization followed by solid-liquid separation. It has proved to be advantageous to pass the amount of gas discharged before or after the partial separation of phenol over a rearrangement unit filled with acidic ion exchanger. This unit is generally operated at higher temperatures than the reaction unit. In this rearrangement unit, under the prevailing conditions, some of the minor components of the BPA production present in the circulation stream are isomerized to BPA, so that the overall yield of BPA can be increased. In the solid-liquid separation, a portion of the contained BPA is obtained as a BPA-phenol adduct crystal and can be supplied to further purification steps. In addition, a filtrate consisting of typically 60 to 90% by weight of phenol, 3 to 12% by weight of BPA and 3 to 18% by weight of secondary components is obtained. From this filtrate, the phenol contained is typically distilled off to <10 wt .-% residual content and the resulting residual resin containing <10 wt .-% phenol, 14 to 80 wt .-% BPA, 20 to 86 wt .-% secondary components of disposal fed.

The processes described for working up the exit stream have the disadvantage that in the ultimately disposed residual resin even larger amounts of phenol are present either as a substance or bound in BPA or the secondary components. The disposal of the residual resin thus leads to a loss of raw materials.

The object of the present invention was therefore to find a process for the treatment of feed streams from a BPA production process, with the phenol is obtained in high purity and high yields.

1. 1. Minimierung von Phenol im Restharz,
2. 2. Minimierung von BPA im Restharz,
3. 3. Minimierung der Restharzmenge,
4. 4. Bereitstellung von Phenol hoher Reinheit (> 99,8 %) und geringen Anteilen an Verunreinigungen (S, Fe, Cl) aus dem Aufarbeitungsprozess mit hohen Ausbeuten,
5. 5. Kontinuierliche Prozessführung bei gleichzeitigem minimierten Einsatz von Apparaten und Energie

Such a method preferably fulfills the following requirements:

1. 1. minimization of phenol in the residual resin,
2. 2. minimization of BPA in the residual resin,
3. 3. minimizing the amount of residual resin,
4. 4. Providing high purity phenol (> 99.8%) and low levels of impurities (S, Fe, Cl) from the high yield process,
5. 5. Continuous process control while minimizing the use of equipment and energy

1. a) den Teilstrom einer Destillationskolonne enthaltend mindestens 5 theoretische Trennstufen zuführt, und
2. b) in der Destillationskolonne Phenol über Kopf abdestilliert, und
3. c) einen ersten Teil des Sumpfprodukts aus dem Prozess ausschleust, und
4. d) einen zweiten Teil des Sumpfprodukts kontinuierlich in einen Verweilzeitbehälter überführt, in dem man die in dem Sumpfprodukt enthaltenen Nebenkomponenten bei Temperaturen von > 190°C und einer hydrodynamischen Verweilzweit von mindestens 120 min in Gegenwart eines sauren Katalysators zumindest teilweise isomerisiert, und anschließend zurück in die Destillationskolonne führt, wobei der Massenstrom des in den Verweilzeitbehälter geführten Teil des Sumpfprodukts mehr als 30% des Massenstroms des in Schritt a) in die Destillationskolonne eingebrachten Teilstroms beträgt.

The invention relates to a process for the continuous separation of phenol from a partial stream produced in the production of bisphenol A containing 40 to 90 wt .-% of phenol, 5 to 40% by weight of bisphenol A and 5 to 40% by weight of secondary components which are formed in the reaction of phenol and acetone to give bisphenol A,
in which one

1. a) supplying the substream to a distillation column containing at least 5 theoretical plates, and
2. b) distilled overhead in the distillation column phenol, and
3. c) discharging a first part of the sump product from the process, and
4. d) continuously converting a second part of the bottoms product into a residence time vessel in which the at least partial isomerization of the minor components contained in the bottom product at temperatures of> 190 ° C. and a hydrodynamic residence time of at least 120 minutes in the presence of an acidic catalyst, and then back into the distillation column leads, wherein the mass flow of the guided into the residence time part of the bottom product is more than 30% of the mass flow of the introduced in step a) in the distillation column substream.

The phenol distilled off in step b) preferably has a purity of> 99.8% by weight.

The technical problem is solved by acid-catalyzed cleavage of the exit stream in a residence time vessel and continuous separation of the phenol via a vacuum column with high separation capacity.

FIG. 1 shows a preferred embodiment of the method according to the invention.

In this case, a part stream 1 fed off from a process for the production of bisphenol A is fed into a distillation column 2 with a high separation capacity. The high separation performance results from the at least 5 theoretical plates. In this distillation column phenol vapors 3 are separated overhead, which are condensed in a cooler 4 to liquid phenol 5. The bottom product or the bottom discharge 6 of the column 2 is circulated via a heat exchanger 7, so that in this way the energy required for the evaporation of phenol is introduced into the system. The cooler 4 and the evaporator 7 may be independent installation parts or may be structurally integrated into the distillation columns. A partial stream 9 of the bottom product is optionally supplied to the disposal after storage in a buffer tank 10 as residual resin. Another partial stream 8 of the bottom product is fed to a residence time tank 11. In this residence time tank, the acid-catalyzed isomerization takes place and cleavage of BPA and minor components to form phenol. The cleavage is accelerated by the addition of suitable acids (stream 12) through a metering unit 13. The phenol-containing effluent 14 of the residence time vessel 11 is combined with the recirculated substream 1 and thus returned to the distillation unit 2 for the recovery of phenol.

• Bevorzugt ist Teilstrom 1 ein ausgespeister Teilstrom aus einem BPA-Herstellprozess mit Suspensionskristallisation und Fest-Flüssigtrennung, wobei ein Teilstrom des Filtrats der Fest-Flüssigtrennung zur Ausspeisung von Nebenprodukten in Richtung des erfindungsgemäßen Verfahrens zur Abtrennung von Phenol abgeführt wird. Optional wird hierbei der genannte Teilstrom 1 vor der Einspeisung in die Destillationskolonne 2 durch eine zusätzliche Umlagerungseinheit geführt, wobei durch Behandeln mit saurem Ionentauscher, anschließendes partielles Abdestillieren von Phenol, Kristallisation und Fest-Flüssigtrennung ein Teil des im Filtrat enthaltenen BPA wiedergewonnen und dem Hauptprozess zugeführt wird. In diesem Fall dient das Filtrat der Fest-Flüssigtrennung der Umlagerungseinheit als Einspeisestrom 1 der Destillationskolonne 2.

The process is operated continuously and reaches a state of equilibrium after a short time. In principle, the partial stream 1 fed out from the process for producing bisphenol A can be any product stream from a process for producing BPA which contains the following components: 40 to 90% by weight of phenol, 5 to 40% by weight of bisphenol A and 5 to 40% by weight of secondary components which are obtained in the reaction of phenol and acetone to give bisphenol A:

• Partial stream 1 is preferably a recycled partial stream from a BPA production process with suspension crystallization and solid-liquid separation, wherein a partial stream of the filtrate of solid-liquid separation for the discharge of by-products in the direction of the process according to the invention for the separation of phenol is removed. Optionally, this partial stream 1 is passed through an additional rearrangement unit before being fed into the distillation column 2, wherein part of the BPA contained in the filtrate is recovered by treatment with acidic ion exchanger, followed by partial distilling off of phenol, crystallization and solid-liquid separation and fed to the main process becomes. In this case, the filtrate of the solid-liquid separation of the rearrangement unit serves as the feed stream 1 of the distillation column 2.

In order to allow the most efficient cleavage, separation and purification of the phenol-containing residues, the following boundary conditions must be observed in the procedure according to the invention: the distillation column 2 must have at least 5 theoretical plates, preferably at least 10 theoretical plates, to separate other low-boiling components such as isopropenylphenol to allow and to ensure a purification of the resulting phenol to a purity of> 99.8 wt .-% at the top of the column. The distillation is preferably operated at an absolute pressure at the top of the column of 70 to 200 mbar, preferably 90 to 120 mbar. The residence time tank 11 must be designed in such a way that an average hydrodynamic residence time of the circulation stream 8 of at least 2 hours, preferably at least 4 hours, is set in order to achieve an effective and as complete as possible cleavage of BPA and by-products. The residence time reactor can in this case be completely filled in upflow or downflow or operated in a controlled manner.

The isomerization and the associated cleavage in the residence time vessel 11 in step d) is carried out in the presence of an acidic catalyst. As acidic catalyst (stream 12) for the cleavage in step d) can in principle a variety of strongly acidic heavy or non-volatile Brönsted acids are used, including phosphoric acid or its higher condensates, sulfuric acid, alkanesulfonic acids having> 4 carbon atoms in the alkane chain, aromatic Sulfonic acids, arylalkane sulfonic acids or phosphonium acids. In addition to these homogeneously used acids, it is also possible to use heterogeneous cleavage catalysts such as, for example, strongly acidic aluminum oxide, supported Lewis acids, acidic zeolites or other clays or polystyrrolesulfonic acids. In this case, the metering unit 13 is omitted and the heterogeneous catalyst is introduced by a suitable retaining structure in the residence time tank 11 and replaced if necessary. In a preferred embodiment of the invention, the isomerization and cleavage in step d) in the presence of sulfuric acid, phosphoric acid or p-toluene sulfonic acid, more preferably with sulfuric acid. The dosage can be as in FIG. 1 shown as stream 12 via the metering unit 13 in the circulation stream of the bottom product of the distillation column 2. However, the metering can also take place in the input stream 8 into the residence time container 11 or into the output stream 14 out of the residence time container 11. Finally, the metering can also be carried out in the partial stream 1 in the distillation column 2. Preferably, the metering is in the circulation stream of the bottom product of the distillation column 2 of the column.

wobei

M

(Schwefelsäure) den Massenstrom an Schwefelsäure in kg/h, und

c

den Konzentrationsfaktor, und

X

den Massenanteil an Phenol im Teilstrom 1, und

M (1)

den Massenstrom des Teilstroms 1 in kg/h bedeuten.

The preferred concentration of the acidic cleavage catalyst depends on the amount of cleavable by-products in the input stream and the type of acid used and can be easily determined in simple experiments. For the preferred embodiment with sulfuric acid, the metered amount of acid results as follows: $$\mathrm{M}\left(\mathrm{sulfuric\; acid}\right)=\mathrm{c}*\left(\mathrm{1}-\mathrm{X}\right)*\mathrm{M}$$

in which

M

(Sulfuric acid) the mass flow of sulfuric acid in kg / h, and

c

the concentration factor, and

X

the mass fraction of phenol in the partial stream 1, and

M (1)

mean the mass flow of the partial flow 1 in kg / h.

Here, c for effective cleavage is between 0.001% and 2%, preferably between 0.005 and 1%, more preferably between 0.01 and 0.2%. For other acids analogous equations with 0.001% <c <5% apply.

The temperature in the residence time vessel 11 must be> 190 ° C., preferably> 200 ° C., for effective isomerization and cleavage. In order to restrict the use of additional apparatuses, the heat input in a preferred embodiment takes place directly through the evaporator 7 of the distillation column 2. This means that the input stream 8 into the residence time vessel 11 is not heated by a separate heat exchanger and the bottom of the column 2 and the dwell tank 11 are operated at the same temperature.

In order to ensure an efficient separation of the phenol from the output stream 14 out of the residence time tank 11 out, it is necessary that the recycle flow over the residence time compared to the partial stream 1 is not too low. Otherwise, a total of only an incomplete cleavage of fissile by-products and a feed still fissile material on the fed-part stream 9, which is associated with a loss of material. Therefore, the mass flow introduced into the residence time vessel 11 is> 30%, preferably> 80%, very particularly preferably> 100% of the mass flow of the partial stream 1 originating from the process for the production of bisphenol A.

In order to allow the acid-catalyzed cleavage to proceed successfully, it is additionally necessary to select the hydrodynamic residence time in the residence time vessel 11 for at least 2 hours, preferably at least 4 hours.

Examples

The following Examples 1 to 7 are used in a pilot plant according to FIG. 1 carried out.

A technical plant for the preparation of bisphenol A supplies after reaction and separation of BPA a fed-part stream 1, which is the distillative workup in the vacuum distillation column 2 is supplied. The partial stream 1 is supplied to the distillation column 2 with a mass flow M (1) and contains phenol in a mass fraction X and other components in a mass fraction (1-X), which essentially BPA and its isomers and various branched and higher condensation products of acetone and Phenol include such as hydroxyphenyl substituted indanes, chromans, trisphenols, and similar fissile products. The composition (without phenol content) is in all examples and comparative examples: p, p-BPA 35-40 wt .-%, o, p-BPA 9 to 12 wt .-%, hydroxyphenylsubstituierte indanes 15 to 19 wt .-%, hydroxyphenyl-substituted chromans 18 to 22% by weight, trisphenol 3 to 5% by weight, other constituents 9 to 12% by weight. This corresponds to a typical composition of a feed stream from a bisphenol A production plant.

Table 1 shows the purity C (5) and the mass flow M (5) of the phenol stream 5 obtained at the top of the column for the various conditions on which Examples 1 to 7 are based. The column 2 used has 20 theoretical plates and was operated with a reflux ratio of about 0.6 and with the pressures that set in adjusting the temperature T (11) at the bottom of the column 2 in the thermodynamic equilibrium. The temperature in the residence time container 11 is equal to the temperature at the bottom of the column 2. The values indicated relate to equilibrium conditions which occur after a few hours in continuous operation of the column.

For optimum recovery of phenol from substream 1, it is desirable to maximize mass flow M (5) and purity C (5) of the separated phenol stream 5 to recover the largest possible amount of high purity phenol. At the same time, it is necessary to minimize the mass flow M (9) of non-recyclable residual resin 9. In Examples 1 and 2 according to the invention, the cleavage of the fissile constituents contained in the partial stream 1 increases the mass flow M (5) of the separated phenol stream 5, so that, based on the mass fraction X of phenol in the partial stream 1, a phenol yield Y = M (5 ) / (X * M (1)) of> 1.20 and at the same time a phenol stream 5 of high purity C (5) is obtained and the mass flow M (9) is minimized at fed-out residual resin 9.

In Examples 1 to 3 and 5 to 7, the cleavage in the residence time vessel 11 is assisted by the addition of sulfuric acid. The quantities of sulfuric acid M (sulfuric acid) used are given by the following equation: M (sulfuric acid) = c * (1-X) * M (1).

In Comparative Example 3, the temperature T (11) in the residence time vessel 11 is lowered to 180 ° C. As a result, the phenol yield drops to 1.03, d. H. there is no effective division anymore.

Comparative Example 4 shows the influence of sulfuric acid concentration. Without feeding in additional acid (c = 0 in Example 4 means M (sulfuric acid) = 0) there is no effective cleavage, the phenol yield therefore drops to 0.97.

Comparative Example 5 shows the effect of the mass flow of the stream of bottom product passed through the residence time vessel 11. Only when there is sufficient mass flow M (8) of the bottom product passed through the residence time vessel 11 and back into the column 2 is effective cleavage and recovery of phenol carried out. For M (8) <0.3 * M (1), the phenol yield therefore drops to 1.05.

Comparative example 6 is carried out under otherwise identical construction with a column of lower separation efficiency (number of theoretical plates = 1). Although a high phenol yield Y = 1.27 is achieved, only a moderate phenol purity (C 5) = 96.2% by weight is achieved at the top of the column.

Comparative Example 7 is carried out with the already in Examples 1 to 5 Column 2, but the residence time 11 is bypassed, so that no residence time for cleavage is available. As a result, despite the addition of sulfuric acid, no high yield of phenol Y in the phenol stream 5 is realized.

The examples and comparative examples show that in the process according to the invention using a distillation column 2 and a residence time 11 when metering an acidic cleavage catalyst phenol of high purity can be obtained in increased yield at the top of the column and at the same time the amount of residual resin can be minimized. <b><u> Table 1 </ u></b> M (1) [t / h] X [-] c [%] T (11) [° C] C (5) [% by weight] M (5) [t / h] Y [-] M (9) [t / h] M (8) [t / h] τ (11) [h] Ex. 1 ^{1) 2)} 2.75 0.65 0.06 205 99.88 2.19 1.22 0.56 3.10 19.4 Ex. 2 ^{1) 2)} 2.40 0.50 0.06 210 99.91 1.72 1.43 0.68 2.95 20.3 Comparative Ex. 3 ^{1) 2)} 2.65 0.62 0.06 180 99.75 1.69 1.03 0.96 2.80 21.4 Comparative Ex. 4 ^{1) 2)} 2.80 0.59 0.0 205 99.87 1.60 0.97 1.20 3.05 19.7 Comparative Ex. 5 ^{1) 2)} 2.75 0.63 0.05 200 99.89 1.82 1.05 0.93 0.5 120.0 Comparative Ex. 6 ^{3) 2)} 2.90 0.62 0.06 205 96.25 2.28 1.27 0.62 3.2 18.8 Comparative Ex. 7 ^{1) 4)} 2.90 0.58 0.05 205 99.58 1.70 1.01 1.20 3.2 0 With
M (1) = mass flow of the partial flow 1,
X = phenol content in substream 1,
C = Concentration factor for sulfuric acid addition M (H ₂ SO ₄ ) = c * (1-X) * M (1); 96% sulfuric acid was metered in,
T (11) = temperature in the residence time container 11,
C (5) = content of phenol in the phenol stream 5,
M (5) = mass flow of phenol stream 5,
Y = phenol yield based on the content of phenol in substream 1: Y = M (5) / (X * M (1)),
M (9) = mass flow of discharged residual resin 9 for disposal,
M (8) = mass flow of bottom product through the residence time tank 11,
τ (11) = hydrodynamic residence time in the residence time vessel (11).
footnotes:
1) a vacuum column with 20 theoretical plates is used
2) a residence time container with a volume of 60 m ³ is used
3) a vacuum column with 1 theoretical plate is used
4) the residence time container 11 is bypassed

Claims (5)

1. Method of continuously separating phenol from a substream produced in the production of bisphenol A and comprising from 40 to 90 wt% of phenol, from 5 to 40 wt% of bisphenol A and from 5 to 40 wt% of secondary components generated in the reaction of phenol and acetone to afford bisphenol A,
   wherein

   a) the substream is supplied to a distillation column comprising at least 5 theoretical plates, and

   b) phenol is distilled off overhead in the distillation column, and

   c) a first portion of the bottoms product is discharged from the process, and

   d) a second portion of the bottoms product is continuously transferred into a delay vessel, in which the secondary components present in the bottoms product are at least partially isomerized in the presence of an acidic catalyst at temperatures of > 190°C with a hydrodynamic residence time of at least 120 min, and subsequently recycled into the distillation column, wherein the mass flow rate of the portion of the bottoms product passed into the delay vessel is greater than 30% of the mass flow rate of the substream introduced into the distillation column in step a).
2. Method according to Claim 1, wherein the acidic catalyst employed in step d) is sulphuric acid.
3. Method according to either of Claims 1 and 2, wherein the isomerization in step d) is carried out at temperatures of > 200°C.
4. Method according to any of Claims 1 to 3, wherein the isomerization in step d) is carried out with a hydrodynamic residence time of at least 4 hours.
5. Method according to any of Claims 1 to 4, wherein step b) affords phenol of > 99.8 wt% purity.

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